Method and device for optical amplification

ABSTRACT

The present invention relates to a device for optical amplification, and a primary object of the present invention is to provide a method and device for optical amplification which can maintain the wavelength characteristic of gain constant and can obtain a wide input dynamic range. The device according to the present invention includes a first optical amplifying unit having a Raman amplifying medium and a first pump source for pumping the Raman amplifying medium, and a second optical amplifying unit optically connected to a rear stage of the first optical amplifying unit. The second optical amplifying unit has an optical amplifying medium and second pump sources for pumping the optical amplifying medium. The present invention is characterized by a control unit for controlling the gain of the first optical amplifying unit so that variations in output power of the first optical amplifying unit due to variations in input power of the first optical amplifying unit are canceled.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method and device for opticalamplification.

[0003] 2. Description of the Related Art

[0004] An optical communication system using an optical fibertransmission line is used to transmit a relatively large amount ofinformation. A low-loss (e.g., 0.2 dB/km) optical fiber has already beenproduced and is being used as the optical fiber transmission line. Inaddition, an optical amplifier for compensating for loss in the opticalfiber transmission line is used to allow long-haul transmission.

[0005] A conventional optical amplifier includes an optical amplifyingmedium pumped by pump light to provide a gain band. The opticalamplifying medium and the pump light are selected so as to provide again band including the wavelength of signal light to be amplified. As aresult, the signal light is amplified during propagation in the opticalamplifying medium being pumped.

[0006] For example, an erbium doped fiber amplifier (EDFA) includes anerbium doped fiber (EDF) as the optical amplifying medium, and a pumpsource for pumping the EDF. The pump source supplies pump light having apredetermined wavelength to the EDF. By presetting the wavelength of thepump light within a 0.98 μm band or 1.48 μm band, a gain band includinga wavelength band of 1.55 μm can be obtained. As a result, signal lighthaving a wavelength band of 1.55 μm is amplified.

[0007] As a technique for increasing a transmission capacity by a singleoptical fiber, wavelength division multiplexing (WDM) is known. In asystem adopting WDM, a plurality of optical carriers having differentwavelengths are individually modulated by data. Each modulated carrierprovides one channel of a WDM system for transmitting optical signals.These optical signals (i.e., the modulated carriers) are wavelengthdivision multiplexed by an optical multiplexer to obtain WDM signallight. The WDM signal light thus obtained is transmitted through anoptical fiber transmission line to a receiving end. At the receivingend, the WDM signal light is separated into individual optical signalsby an optical demultiplexer. Then, the original data can be detectedaccording to these individual optical signals. Accordingly, by applyingWDM, the transmission capacity in a single optical fiber can beincreased according to the number of WDM channels.

[0008] In the case that an optical amplifier is inserted in atransmission line of an optical communication system adopting WDM, atransmission distance is limited by the noise characteristic of theoptical amplifier and by the wavelength characteristic of gain which isrepresented by a gain tilt or gain deviation. In an EDFA, for example, apractical amplification band is present in a band of 1530 to 1610 nm,and a gain tilt is produced near this amplification band. It is knownthat this gain tilt varies with the total input power of signal light tothe EDFA and the power of pump light.

[0009] To compensate the wavelength characteristic of gain of an opticalamplifier, an optical filter is used as an equalizer. However, when theaverage gain of the optical amplifier is changed, the wavelengthcharacteristic of gain largely changes. It is therefore necessary toperform control such that the gain becomes constant at a gain pointwhere the optical filter is fabricated.

[0010] In an optical communication system, optical fiber transmissionlines of various lengths are provided. Accordingly, the power of signallight to be input into an optical amplifier used as an optical repeateris not constant. It is therefore required to provide an opticalamplifier which can support a wide input power dynamic range. Further,there is a case that a dispersion compensating fiber for compensatingdispersion generated in an optical fiber transmission line is used. Inthis case, it is also necessary to change a level diagram in an opticalrepeater according to variations in loss in the dispersion compensatingfiber.

[0011] In such gain control that the gain of an optical amplifierbecomes constant as mentioned above, the power of output signal lightchanges with a change in the power of input signal light. However, thepower of output signal light allowed to be supplied to an optical fibertransmission line downstream of the optical amplifier is limited byvarious nonlinear effects. Therefore, it is desirable that the outputpower from the optical amplifier is constant.

[0012] In these circumstances, there has been developed an opticalamplifying device for maintaining the wavelength characteristic of gainconstant and obtaining a wide input dynamic range. This opticalamplifying device includes first and second optical amplifiers and avariable optical attenuator optically connected between the first andsecond optical amplifiers. Automatic gain control (AGC) is applied toeach of the first and second optical amplifiers, thereby maintainingconstant the wavelength characteristic of gain of each of the first andsecond optical amplifiers. Further, automatic output level control (ALC)is performed to the second optical amplifier by using the variableoptical attenuator to thereby obtain a wide input dynamic range. Thatis, the output level of the second optical amplifier is maintainedconstant irrespective of the input level of the first optical amplifier,so that the input dynamic range of this optical amplifying device iswidened.

[0013] However, this type of optical amplifying device isdisadvantageous from the viewpoints of efficiency and noisecharacteristics, because undue loss is given by the variable opticalattenuator. Particularly in the case that the amount of dispersioncompensation is large, a dispersion compensating fiber having a lengthof ten and more kilometers is necessary. In this case, it is consideredthat a loss of about 20 dB may be incurred as the sum of the loss by thedispersion compensating fiber and the input dynamic range, causing alarge degradation in efficiency of an amplifier repeater.

[0014] Further, with higher transmission speeds in recent years, theinfluences by nonlinear effects (self-phase modulation, cross-phasemodulation, etc.) have become apparent as the cause of degradation inerror rate. Particularly in the case that the transmission speed is 40Gbit/s or more, it is necessary to minimize the influences by thenonlinear effects to signal light. In a repeater, the nonlinearitycaused in a dispersion compensating fiber is large, so that it isnecessary to reduce the power of signal light to be input into thedispersion compensating fiber, resulting in a degradation in noisefigure in the repeater.

[0015] Further, there has been proposed a Raman amplifier having a Ramanamplifying medium and a plurality of pump sources for pumping the Ramanamplifying medium at different wavelengths, for the purpose ofbroadening the amplification band for signal light. As a method ofcontrolling the wavelength characteristic of gain in the Ramanamplifier, it is known that a monitor for monitoring the wavelengthcharacteristic of optical transmission power of signal light passedthrough the Raman amplifying medium is used to feed back the result ofthis monitoring to each pump source, thereby controlling the wavelengthcharacteristic of gain (Japanese Patent Laid-open No. 2001-15845).

SUMMARY OF THE INVENTION

[0016] In this method, however, it is necessary to correct for the powerof interchannel crosstalk and spontaneous Raman scattering light in themonitored optical power, so there is a problem in control. Furthermore,it is necessary to use a spectrum analyzer for monitoring the signallight with high accuracy, inviting a disadvantage from the viewpoint ofcost.

[0017] It is therefore an object of the present invention to provide amethod and device for optical amplification which can maintain thewavelength characteristic of gain constant and can obtain a wide inputdynamic range, thereby improving the amplification efficiency and noisecharacteristics. Other objects of the present invention will becomeapparent from the following description.

[0018] In accordance with an aspect of the present invention, there isprovided a device comprising a first optical amplifying unit, a secondoptical amplifying unit, and a control unit. The first opticalamplifying unit comprises a Raman amplifying medium and a first pumpsource for pumping the Raman amplifying medium. The second opticalamplifying unit is optically connected to a rear stage of the firstoptical amplifying unit, and comprises an optical amplifying medium anda second pump source for pumping the optical amplifying medium. Thecontrol unit controls the gain of the first optical amplifying unit sothat variations in output power of the first optical amplifying unit dueto variations in input power of the first optical amplifying unit arecanceled.

[0019] With this configuration, variations in output power of the firstoptical amplifying unit are canceled, so that the gain of the secondoptical amplifying unit can be maintained constant. Accordingly, thewavelength characteristic of gain of the whole device can be easilymaintained constant, and a wide input dynamic range can be obtained.Moreover, the attenuation of a variable optical attenuator that may besometimes provided between the first optical amplifying unit and thesecond optical amplifying unit can be reduced or nullified by the abovecontrol, thereby allowing the improvement in amplification efficiencyand in noise characteristics.

[0020] In accordance with another aspect of the present invention, thereis provided a method for optical amplification. This method is a methodusing the device according to the present invention. More specifically,the method according to the present invention comprises the steps ofamplifying signal light by a first optical amplifying unit comprising aRaman amplifying medium and a first pump source for pumping the Ramanamplifying medium; amplifying signal light output from the first opticalamplifying unit by a second optical amplifying unit comprising anoptical amplifying medium and a second pump source for pumping theoptical amplifying medium; and controlling the gain of the first opticalamplifying unit so that variations in output power of the first opticalamplifying unit due to variations in input power of the first opticalamplifying unit are canceled.

[0021] The above and other objects, features and advantages of thepresent invention and the manner of realizing them will become moreapparent, and the invention itself will best be understood from a studyof the following description and appended claims with reference to theattached drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a block diagram showing a first preferred embodiment ofthe device for optical amplification according to the present invention;

[0023]FIG. 2 is a level diagram in the device shown in FIG. 1;

[0024]FIG. 3A is a block diagram showing the configuration of aconventional optical amplifier, and

[0025]FIG. 3B is a level diagram in the optical amplifier shown in FIG.3A;

[0026]FIG. 4 is a block diagram showing a second preferred embodiment ofthe device for optical amplification according to the present invention;

[0027]FIG. 5 is a block diagram showing a specific configuration of again tilt control loop 26 shown in FIG. 4;

[0028]FIG. 6 is a diagram for illustrating the passbands of bandpassfilters 31 shown in FIG. 5;

[0029]FIG. 7 is a block diagram showing a third preferred embodiment ofthe device for optical amplification according to the present invention;

[0030]FIG. 8A is a graph showing the wavelength characteristic of outputpower in the preferred embodiment shown in FIG. 1, and FIG. 8B is agraph showing the wavelength characteristic of output power in thepreferred embodiment shown in FIG. 7;

[0031]FIG. 9 is a block diagram showing a fourth preferred embodiment ofthe device for optical amplification according to the present invention;

[0032]FIG. 10 is a block diagram showing a fifth preferred embodiment ofthe device for optical amplification according to the present invention;

[0033]FIG. 11 is a block diagram showing a sixth preferred embodiment ofthe device for optical amplification according to the present invention;

[0034]FIG. 12 is a block diagram showing a seventh preferred embodimentof the device for optical amplification according to the presentinvention;

[0035]FIG. 13 is a block diagram of a system to which the presentinvention is applicable;

[0036]FIG. 14 is a block diagram showing an experimental device forverification of the effectiveness of the present invention;

[0037]FIG. 15 is a graph showing the wavelength characteristics of gainand NF in an EDFA shown in FIG. 14; and

[0038]FIG. 16 is a graph showing the wavelength characteristics of gainand NF in the whole device and a DCFRA shown in FIG. 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Some preferred embodiments of the present invention will now bedescribed in detail with reference to the attached drawings.

[0040] Referring to FIG. 1, there is shown a first preferred embodimentof the device for optical amplification according to the presentinvention. This device includes a Raman amplifying medium 1, opticalcoupler 2, pump source 3, optical isolator 4, pump source 5, opticalcoupler 6, EDF (erbium doped fiber as a rare earth doped fiber) 7, pumpsource 8, optical coupler 9, and optical isolator 10. The Ramanamplifying medium 1 has a first end connected to an input port 11 and asecond end connected to the optical coupler 2. The pump source 3 and aninput end of the optical isolator 4 are also connected to the opticalcoupler 2. An output end of the optical isolator 4 and the pump source 5are connected to the optical coupler 6. The EDF 7 has a first endconnected to the optical coupler 6 and a second end connected to theoptical coupler 9. The pump source 8 and an input end of the opticalisolator 10 are also connected to the optical coupler 9. An output endof the optical isolator 10 is connected to an output port 12.

[0041] A dispersion compensating fiber or a highly nonlinear fiber maybe used as the Raman amplifying medium 1. In the case of using adispersion compensating fiber as the Raman amplifying medium 1, thedispersion compensating fiber compensates for the chromatic dispersionand dispersion slope of an optical fiber transmission line.

[0042] The wavelength of signal light to be amplified is selected fromthe S-band (1450 to 1530 nm), C-band (1530 to 1570 nm), and L-band (1570to 1610 nm), for example. In each case, the wavelength of pump lightfrom the pump source 3 is set shorter than the band of the signal lightby about 100 nm (about 13 THz). The wavelength of the pump light fromthe pump source 3 may be single or multiple.

[0043] The signal light to be amplified propagates from the input port11 toward the output port 12. The pump light output from the pump source3 is supplied through the optical coupler 2 into the Raman amplifyingmedium 1 from its second end and propagates toward the first end of theRaman amplifying medium 1. In the case that the wavelength of the pumplight from the pump source 3 is multiple, each wavelength can provide aRaman amplification band at a wavelength longer by a Raman Stokes shift,thereby broadening the amplification band.

[0044] The wavelength of pump light from the pump source 5 is set to980±5 nm, for example. The pump light output from the pump source 5 issupplied through the optical coupler 6 into the EDF 7 from its first endand propagates toward the second end of the EDF 7. The wavelength ofpump light from the pump source 8 is set to 1480±10 nm, for example. Thepump light output from the pump source 8 is supplied through the opticalcoupler 9 into the EDF 7 from its second end and propagates toward thefirst end of the EDF 7.

[0045] The EDF 7 as a rare earth doped fiber is suitable foramplification of signal light in the C-band. The rare earth element as adopant in the rare earth doped fiber may be selected according to thewavelength of signal light to be amplified.

[0046] In this preferred embodiment, the Raman amplifying medium 1, theoptical coupler 2, and the pump source 3 constitute a first opticalamplifying unit 13. The pump source 5, the optical coupler 6, the EDF 7,the pump source 8, and the optical coupler 9 constitute a second opticalamplifying unit 14. The optical isolators 4 and 10 are provided toprevent the oscillation due to a reflector structure formed in anoptical path including an optical amplifying medium.

[0047] When the power of signal light supplied to the input port 11varies, the gain in the first optical amplifying unit 13 is controlledto thereby maintain constant the power of signal light to be supplied tothe second optical amplifying unit 14. The second optical amplifyingunit 14 is used to obtain a high output power. It is preferable toperform gain fixing control so that the wavelength dependence of gain isnot changed.

[0048] Particularly in the case of amplifying wavelength divisionmultiplexing (WDM) signal light, it is preferable that multiplewavelengths are used as the operating wavelength of the pump source 3 tocontrol the output level of each optical signal of the WDM signal light,thereby maintaining the level of each optical signal to be input to thesecond optical amplifying unit 14 at a predetermined constant level.

[0049] The predetermined constant level mentioned above means that theoutput levels in all the wavelength channels are equal to each other orthat there is a level deviation between the wavelength channels.However, even in the case that the level deviation is present, the levelof each optical signal to be input to the second optical amplifying unit14 is preferably controlled to become constant irrespective ofvariations in input level of signal light to the first opticalamplifying unit 13.

[0050] Referring to FIG. 2, there is shown a level diagram in the deviceshown in FIG. 1. The level diagram indicates that variations in power ofsignal light input to this device are absorbed by gain control of thefirst optical amplifying unit 13 and that the second optical amplifyingunit 14 is operated under gain fixing control. In the case that theoptical fiber transmission line is an SMF (single-mode fiber) having alength of 100 km, the length of a dispersion compensating fiber requiredfor a repeater becomes about 20 km. In the case that this dispersioncompensating fiber is used as the Raman amplifying medium 1 (see FIG.1), a pump light power of about 400 mW is required to support a loss(about 12 dB) of the dispersion compensating fiber and an input dynamicrange of 7 dB (C-band, 44 channels, input power of −17 to −10 dBm/ch).To the contrary, a pump light power of about 800 mW at the maximum issometimes required in the prior art. This will now be described morespecifically.

[0051] Referring to FIG. 3A, there is shown a block diagram of a priorart device for optical amplification. This device is configured bycascading an EDFA (erbium doped fiber amplifier) 15, an optical variableattenuator 16, and an EDFA 17.

[0052]FIG. 3B shows a level diagram in the device shown in FIG. 3A. Ineach of the EDFAs 15 and 17, gain fixing control is performed tomaintain the wavelength characteristic of gain constant. As a result,the output power of the EDFA 15 becomes 9 dBm/ch, and a pump light powerof about 800 mW at the maximum is therefore required.

[0053] Thus, the pump light power can be reduced to a substantially halfvalue according to the preferred embodiment shown in FIG. 1 as comparedwith that in the prior art, thereby greatly reducing the cost of thepump source 3 in the first optical amplifying unit 13.

[0054] Referring to FIG. 4, there is shown a second preferred embodimentof the device for optical amplification according to the presentinvention. In contrast to the preferred embodiment shown in FIG. 1, thepreferred embodiment shown in FIG. 4 is characterized in that aspontaneous Raman scattering light monitor unit 18, optical couplers 19,20, and 21, power monitors 22 and 23, AGC loop 24, ALC loop 25, and gaintilt control loop 26 are additionally provided.

[0055] The optical coupler 19 is used to branch off spontaneous Ramanscattering light generated from the second end of the Raman amplifyingmedium 1 toward the first end thereof, from the main optical path. Thespontaneous Raman scattering light branched off is supplied to themonitor unit 18. The monitor unit 18 includes an optical demultiplexer27 for separating the Raman scattering light into a plurality ofdifferent wavelength components, and a plurality of power monitors 28for detecting the powers of the wavelength components output from theoptical demultiplexer 27. The number of the power monitors 28 is setequal to the number of wavelengths of the pump light in the pump source3.

[0056] The optical coupler 20 is used to branch off a part of signallight to be supplied from the first optical amplifying unit 13 to thesecond optical amplifying unit 14. The signal light branched off by theoptical coupler 20 is supplied to the power monitor 22. The opticalcoupler 21 is used to branch off a part of signal light output from thesecond optical amplifying unit 14. The signal light branched off by theoptical coupler 21 is supplied to the power monitor 23. Accordingly, thegain in the second optical amplifying unit 14 can be calculatedaccording to the outputs from the power monitors 22 and 23. The AGC loop24 controls the second optical amplifying unit 14 according to the gaincalculated above so that the gain in the second optical amplifying unit14 becomes constant. More specifically, the AGC loop 24 controls thepowers (power) of the pump sources 5 and/or 8 (see FIG. 1) in the secondoptical amplifying unit 14.

[0057] The ALC loop 25 controls the pump source 3 so that the outputfrom the power monitor 22 becomes constant. More specifically, the ALCloop 25 controls the total output power in the pump source 3.

[0058] The gain tilt control loop 26 controls the pump source 3according to the outputs from the spontaneous Raman scattering lightmonitor unit 18. More specifically, the gain tilt control loop 26controls the power balance among the plural wavelengths in the pumpsource 3 so that the gain tilt calculated according to data obtained inthe monitor unit 18 becomes constant.

[0059] Accordingly, the ALC loop 25 constitutes a control unit in thispreferred embodiment.

[0060] Referring to FIG. 5, there is shown a specific configuration ofthe gain tilt control loop 26. In this configuration, an opticalcirculator 29 is used in place of the optical coupler 19.

[0061] The spontaneous Raman scattering light output from the opticalcirculator 29 is divided into three outputs by an optical branch unit30. These three outputs are supplied to three bandpass filters 31 havingdifferent transmission center wavelengths, respectively. The outputsfrom the three bandpass filters 31 are supplied to three power monitors28, respectively. In each power monitor 28, the optical input isconverted into an electrical signal. The optical branch unit 30 and thebandpass filters 31 constitute the optical demultiplexer 27 (see FIG.4).

[0062] The pump source 3 is composed of three LDs (laser diodes) 32 andan optical multiplexer (MUX) 33 for combining the outputs from the threeLDs 32. The output from the optical multiplexer 33 is supplied throughthe optical coupler 2 to the Raman amplifying medium 1.

[0063] A tilt control circuit unit 34 is provided to control the balanceof drive currents for the LDs 32 according to the outputs from the powermonitors 28 so that the outputs from the power monitors 28 becomesubstantially the same value.

[0064] The passband of each bandpass filter 31 shown in FIG. 5 will nowbe described with reference to FIG. 6. The spectrum of the backwardspontaneous Raman scattering light has three peaks whose wavelengths arerespectively shifted from the operating wavelengths of the three LDstoward longer wavelengths by about 13.2 THz. Accordingly, the passbandsof the three bandpass filters 31 are set so as to coincide with thethree peaks, respectively. Accordingly, the tilt control circuit unit 34controls the LDs 32 according to the outputs from the power monitors 28so that they become constant as described above with reference to FIG.5, thereby maintaining constant the gain tilt in the first opticalamplifying unit 13.

[0065] The diagram shown on the upper side in FIG. 6 is an example ofthe output spectrum of the first optical amplifying unit 13. The diagramshown on the down side in FIG. 6 is an example of the backwardspontaneous Raman scattering light spectrum. The band-pass filter 31extracts light having wavelengths included in the gain peak band of thepumping LDs 32.

[0066] Referring to FIG. 7, there is shown a third preferred embodimentof the device for optical amplification according to the presentinvention. In contrast to the preferred embodiment shown in FIG. 1, thepreferred embodiment shown in FIG. 7 is characterized in that a variableoptical attenuator 35 is additionally provided between the first opticalamplifying unit 13 and the second optical amplifying unit 14.

[0067] In the preferred embodiment shown in FIG. 1, when the power ofsignal light to be supplied to the input port 11 changes, the ripple inthe output spectrum of the first optical amplifying unit 13 increasesaccording to variations in gain of the first optical amplifying unit 13.More specifically, when the input power of signal light increases by 10dB/ch, a ripple of about 0.7 dB is generated.

[0068] In the preferred embodiment shown in FIG. 7, when the gaincontrol amount in the first optical amplifying unit 13 reaches a givenvalue, variations in the input power of signal light are absorbed by thevariable optical attenuator 35. Accordingly, the power of signal lightto be input to the second optical amplifying unit 14 can be maintainedconstant.

[0069]FIGS. 8A and 8B show the output spectra in the preferredembodiments shown in FIGS. 1 and 7, respectively. In each figure, thevertical axis represents output power and the horizontal axis representswavelength. In the preferred embodiment shown in FIG. 1, the ripple inthe output spectrum in the case of variations in signal input power isrelatively large. In contrast thereto, it is apparent from FIG. 8B thatin the preferred embodiment shown in FIG. 7 the ripple in the outputspectrum is relatively small even when the same variations in signalinput power occur. Thus, the preferred embodiment shown in FIG. 7 canmeet the requirement that an interchannel deviation of repeater outputis to be suppressed to a given value or less.

[0070] In each of the preferred embodiments shown in FIGS. 1 and 7, whenthe signal input power is a reference value, almost no ripples aregenerated.

[0071] Referring to FIG. 9, there is shown a fourth preferred embodimentof the device for optical amplification according to the presentinvention. This preferred embodiment is characterized in that the firstoptical amplifying unit 13 is divided into a front stage 36 and a rearstage 37. The rear stage 37 has the same configuration as that of thefirst optical amplifying unit 13 shown in FIG. 1. The front stage 36includes a Raman amplifying medium 38, an optical coupler 39, and a pumpsource 40 respectively corresponding to the Raman amplifying medium 1,the optical coupler 2, and the pump source 3. The Raman amplifyingmedium 38 has a first end connected to the input port 11 and a secondend connected to the optical coupler 39. Pump light from the pump source40 is supplied through the optical coupler 39 into the Raman amplifyingmedium 39 from its second end and propagates toward the first end of theRaman amplifying medium 38. An optical isolator 41 is provided betweenthe front stage 36 and the rear stage 37. The optical isolator 41 has aninput end connected to the optical coupler 39 and an output endconnected to the first end of the Raman amplifying medium 1.

[0072] A dispersion compensating fiber may be used as either or both ofthe Raman amplifying media 1 and 38. In the case that a dispersioncompensating fiber is used as both of the Raman amplifying media 1 and38, the degree of freedom to the compensation for the chromaticdispersion and dispersion slope in an optical fiber transmission linecan be enlarged. Further, since the first optical amplifying unit 13 isdivided into the front stage 36 and the rear stage 37 in this preferredembodiment, not only the pump source 3 but also the pump source 40 canbe used as a control object in the case of applying the gain tiltcontrol loop 26 and the ALC loop 25 shown in FIG. 4 to the preferredembodiment shown in FIG. 9, so that two independent controls can beeasily performed.

[0073] Referring to FIG. 10, there is shown a fifth preferred embodimentof the device for optical amplification according to the presentinvention. In contrast to the preferred embodiment shown in FIG. 9, thepreferred embodiment shown in FIG. 10 is characterized in that avariable optical attenuator 42 is additionally provided between thefront stage 36 of the first optical amplifying unit 13 and the opticalisolator 41. According to this preferred embodiment, not only the effectobtained by the preferred embodiment shown in FIG. 9, but also theeffect of suppressing the ripple in the output spectrum as describedwith reference to FIG. 7 can be obtained.

[0074] Referring to FIG. 11, there is shown a sixth preferred embodimentof the device for optical amplification according to the presentinvention. In contrast to the preferred embodiment shown in FIG. 9, thepreferred embodiment shown in FIG. 11 is characterized in that adispersion compensating fiber 45 is used as the Raman amplifying medium1 and that a positive dispersion fiber 43 and a negative dispersionfiber 44 cascaded thereto are used as the Raman amplifying medium 38.

[0075] The positive dispersion fiber 43 and the negative dispersionfiber 44 give positive chromatic dispersion and negative chromaticdispersion to signal light, respectively. The lengths of the positivedispersion fiber 43 and the negative dispersion fiber 44 are set so thatthe chromatic dispersion and dispersion slope in the front stage 36 ofthe first optical amplifying unit 13 become zero and that a requiredgain is obtained.

[0076] When the transmission speed per channel becomes tens of gigabitsper second or more, the chromatic dispersion and dispersion slope in anoptical fiber transmission line must be compensated with high accuracy.According to this preferred embodiment, the chromatic dispersion in thefront stage 36 is adjusted to 0 ps/nm, thereby eliminating the need forgiving an extra chromatic dispersion to the dispersion compensatingfiber 45 for compensating the dispersion in the optical fibertransmission line. As a result, the device can be easily designed.

[0077] Referring to FIG. 12, there is shown a seventh preferredembodiment of the device for optical amplification according to thepresent invention. This preferred embodiment is characterized in that anupstream optical fiber transmission line 46 is utilized as a part of adistribution constant type Raman amplifier.

[0078] To this end, pump light output from a pump source 47 is suppliedthrough an optical coupler 48 into the optical fiber transmission line46 from its output end. In general, an SMF (single-mode fiber) or NZ-DSF(nonzero-dispersion shifted fiber) is used as the optical fibertransmission line 46. The pump light from the pump source 47 is setaccording to the kind of the optical fiber transmission line 46 and theband of signal light. By supplying the pump light having a suitablewavelength into the optical fiber transmission line 46 from its outputend, the signal light propagating in the optical fiber transmission line46 in a direction opposite to the propagation direction of the pumplight is amplified. The signal light amplified in the optical fibertransmission line 46 is next supplied through the optical coupler 48into an optical demultiplexer (DMUX) 49. By using the opticaldemultiplexer 49, the signal light can be divided into three bands ofthe S-band, C-band, and L-band, for example. Then, the devices foroptical amplification according to the present invention can be appliedin parallel to these three bands.

[0079] In this preferred embodiment, the rear stage 37 of the firstoptical amplifying unit 13 and the second optical amplifying unit 14shown in FIG. 11 are applied to one of the divided bands.

[0080] Accordingly, a difference in loss according to the length andkind of the optical fiber transmission line 46 can be absorbed by eitherthe Raman amplifier including the optical fiber transmission line 46 orthe rear stage 37 or both of them, so that the input power of signallight to be supplied to the second optical amplifying unit 14 can bemaintained constant. Accordingly, the Raman amplifier including theoptical fiber transmission line 46 is considered to correspond to thefront stage 36 of the first optical amplifying unit 13 shown in FIG. 11.

[0081] Referring to FIG. 13, there is shown a preferred embodiment of asystem to which the present invention is applicable. This systemincludes an optical transmitter 50, an optical receiver 51, an opticalfiber transmission line 52 connecting the optical transmitter 50 and theoptical receiver 51, and at least one optical repeater 53 arranged alongthe optical fiber transmission line 52. In this preferred embodiment,optical amplification of WDM signal light in each of the S-band, C-band,and L-band is described.

[0082] In the optical transmitter 50, a plurality of laser diodes 54 areallocated to optical signals in each band, and the optical signals fromthe laser diodes 54 are wavelength division multiplexed by an opticalmultiplexer 55 for each band to obtain WDM signal light. The WDM signallight from each optical multiplexer 55 is amplified by an opticalamplifier 56 to enter an optical multiplexer 57. In the opticalmultiplexer 57, the WDM signal light in the S-band, the WDM signal lightin the C-band, and the WDM signal light in the L-band are wavelengthdivision multiplexed to obtain WDM signal light including the S-band,C-band, and L-band. The WDM signal light from the optical multiplexer 57is transmitted to the optical repeater 53 by the optical fibertransmission line 52.

[0083] In the optical repeater 53, the WDM signal light received isdivided into the WDM signal light in the S-band, the WDM signal light inthe C-band, and the WDM signal light in the L-band by an opticaldemultiplexer 58. The WDM signal light in each band is amplified by anoptical amplifier 59 to which the present invention is applicable, nextentering an optical multiplexer 60. In the optical multiplexer 60, theWDM signal light in the S-band, the WDM signal light in the C-band, andthe WDM signal light in the L-band are wavelength division multiplexedagain to obtain WDM signal light. The WDM signal light from the opticalmultiplexer 60 is transmitted to the optical receiver 51 by the opticalfiber transmission line 52.

[0084] In the optical receiver 51, the WDM signal light received isdivided into the WDM signal light in the S-band, the WDM signal light inthe C-band, and the WDM signal light in the L-band by an opticaldemultiplexer 61. The WDM signal light in each band is amplified by anoptical amplifier 62, and is next divided into the individual opticalsignals by an optical demultiplexer 63 for each band. Each opticalsignal is converted into an electrical signal by a photodetector 64 toregenerate transmitted data.

[0085] In this preferred embodiment, the present invention is applicableto each of the S-band, C-band, and L-band in the optical repeater 53.Accordingly, the wavelength characteristic of gain can be maintainedconstant and a wide input dynamic range can be obtained in a greatlywide band.

[0086] Referring to FIG. 14, there is shown a block diagram of anexperimental device for verification of the effectiveness of the presentinvention. A DCFRA (dispersion compensating fiber Raman amplifier) asthe first optical amplifying unit 13 and an S-band EDFA (erbium dopedfiber amplifier for the S-band) corresponding to the second opticalamplifying unit 14 are provided between the input port 11 and the outputport 12. In the DCFRA, two DCFs (dispersion compensating fibers) areused as the Raman amplifying medium, and two pumping LDs (pumping laserdiodes) are used as the pump sources for backward pumping the two DCFs,respectively. In the EDFA, two 0.98-μm LDs are used as the pump sourcesfor forward and backward pumping an EDF. Further, optical isolators aresuitably provided in necessary paths.

[0087] In the EDFA, a plurality of (e.g., four) ASE suppressing filterseach for blocking ASE (amplified spontaneous emission noise) generatedin the C-band and transmitting pump light in the 0.98-μm band arearranged along the EDF. With this configuration, optical amplificationfor the S-band with the use of the EDF can be performed.

[0088] In the DCFRA, a desired wavelength characteristic of gain can beobtained by adjusting power distribution at each pumping wavelength, andan input dynamic range can be absorbed by adjusting the total pumppower. In the EDFA, the number of stages of the EDF is preferably set toan optimum value according to the gain of the EDFA. For example, theoptimum number of stages of the EDF in the case of obtaining a gain ofabout 20 dB is 5 from the viewpoints of efficiency and noise.

[0089] Referring to FIG. 15, there are shown the wavelengthcharacteristics of gain and NF (noise figure) in the EDFA shown in FIG.14. As apparent from FIG. 14, the gain has a wavelength dependence of 15dB, and the noise figure has a wavelength dependence of 10 dB. Tocompensate for these wavelength dependences in the case of amplifyingsignal light by using the EDFA only, the pump power and its distributionin the DCFRA were adjusted in this experiment.

[0090] Referring to FIG. 16, there are shown the wavelengthcharacteristics of gain and NF in the whole device and the DCFRA shownin FIG. 14. In FIG. 16, “Hybrid gain” represents the wavelengthcharacteristic of gain in the whole device, “DCFRA gain” represents thewavelength characteristic of gain in the DCFRA only, “Hybrid NF”represents the wavelength characteristic of NF in the whole device, and“DCFRA NF” represents the wavelength characteristic of NF in the DCFRAonly.

[0091] By effectively combining the DCFRA and the EDFA, the wavelengthcharacteristic of gain in the whole device can be suppressed to about 5dB, and the wavelength characteristic of NF in the whole device can bemade substantially flat.

[0092] The configuration of the Raman scattering light monitor on thesignal light input side of the first optical amplifying unit 13 shown inFIG. 4 and the configuration shown in FIG. 5 may be used as a monitorfor gain control of the first optical amplifying unit 13 shown in FIG.7, the first optical amplifying unit 13 (the front stage 36 and the rearstage 37) shown in FIG. 9, the first optical amplifying unit 13 (thefront stage 36 and the rear stage 37) shown in FIG. 10, the firstoptical amplifying unit 13 (the front stage 36 and the rear stage 37)shown in FIG. 11, the first optical amplifying unit 13 (the front stage36 (the portion consisting of the optical fiber transmission line 46,the pump source 47, and the optical coupler 48) and the rear stage 37)shown in FIG. 12, and the DCFRA shown in FIG. 14.

[0093] According to the present invention as described above, it ispossible to maintain the wavelength characteristic of gain constant andobtain a wide input dynamic range. Furthermore, in this condition, theattenuation of a variable optical attenuator that may be sometimesprovided in the device for optical amplification can be reduced ornullified, thereby allowing the improvement in amplification efficiencyand in noise characteristics.

[0094] The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

What is claimed is:
 1. A device comprising: a first optical amplifyingunit comprising a Raman amplifying medium and a first pump source forpumping said Raman amplifying medium; a second optical amplifying unitoptically connected to a rear stage of said first optical amplifyingunit, said second optical amplifying unit comprising an opticalamplifying medium and a second pump source for pumping said opticalamplifying medium; and a control unit for controlling the gain of saidfirst optical amplifying unit so that variations in output power of saidfirst optical amplifying unit due to variations in input power of saidfirst optical amplifying unit are canceled.
 2. A device according toclaim 1, wherein said Raman amplifying medium comprises an opticalfiber, and said optical amplifying medium comprises an erbium dopedfiber.
 3. A device according to claim 1, wherein said first pump sourceoutputs a plurality of pump lights having different wavelengths, therebybroadening the amplification band of said first optical amplifying unit.4. A device according to claim 3, further comprising means for detectingthe spectrum of spontaneous Raman scattering light generated in saidRaman amplifying medium, and means for controlling the balance of thepowers of said plurality of pump lights according to the spectrum ofsaid spontaneous Raman scattering light detected by said detectingmeans.
 5. A device according to claim 3, wherein said control unitcomprises means for detecting the power of light output from said firstoptical amplifying unit, and means for controlling the total power ofsaid plurality of pump lights so that the power detected by saiddetecting means becomes constant.
 6. A device according to claim 1,further comprising means for detecting the optical input power andoptical output power of said second optical amplifying unit, and meansfor controlling said second pump source according to the optical inputpower and optical output power detected by said detecting means so thatthe gain of said second optical amplifying unit becomes constant.
 7. Adevice according to claim 1, further comprising a variable opticalattenuator connected between said first optical amplifying unit and saidsecond optical amplifying unit.
 8. A device according to claim 1,wherein said Raman amplifying medium comprises first and second Ramanamplifying media cascaded to each other, and said first pump sourcecomprises first and second light sources for pumping said first andsecond Raman amplifying media, respectively.
 9. A device according toclaim 8, wherein said first Raman amplifying medium comprises a positivedispersion fiber and a negative dispersion fiber cascaded to each other,and said second Raman amplifying medium comprises a dispersioncompensating fiber.
 10. A device according to claim 8, wherein saidfirst Raman amplifying medium comprises a part or the whole of anoptical fiber transmission line.
 11. A device according to claim 1,wherein said optical amplifying medium comprises a rare earth dopedfiber.
 12. A device according to claim 11, wherein said rare earth dopedfiber contains at least one rare earth element selected from Er, Nd, Tm,Pr, Yb, and Dy.
 13. A method comprising the steps of: amplifying signallight by a first optical amplifying unit comprising a Raman amplifyingmedium and a first pump source for pumping said Raman amplifying medium;amplifying signal light output from said first optical amplifying unitby a second optical amplifying unit comprising an optical amplifyingmedium and a second pump source for pumping said optical amplifyingmedium; and controlling the gain of said first optical amplifying unitso that variations in output power of said first optical amplifying unitdue to variations in input power of said first optical amplifying unitare canceled.
 14. A method according to claim 13, wherein said Ramanamplifying medium comprises an optical fiber, and said opticalamplifying medium comprises an erbium doped fiber.
 15. A methodaccording to claim 13, wherein said first pump source outputs aplurality of pump lights having different wavelengths, therebybroadening the amplification band of said first optical amplifying unit.16. A method according to claim 15, further comprising the steps ofdetecting the spectrum of spontaneous Raman scattering light generatedin said Raman amplifying medium, and controlling the balance of thepowers of said plurality of pump lights according to the spectrum ofsaid spontaneous Raman scattering light detected in said detecting step.17. A method according to claim 15, wherein said controlling stepcomprises the steps of detecting the power of light output from saidfirst optical amplifying unit, and controlling the total power of saidplurality of pump lights so that the power detected in said detectingstep becomes constant.
 18. A method according to claim 13, furthercomprising the steps of detecting the optical input power and opticaloutput power of said second optical amplifying unit, and controllingsaid second pump source according to the optical input power and opticaloutput power detected in said detecting step so that the gain of saidsecond optical amplifying unit becomes constant.
 19. A method accordingto claim 13, further comprising the step of providing a variable opticalattenuator connected between said first optical amplifying unit and saidsecond optical amplifying unit.
 20. A method according to claim 13,wherein said Raman amplifying medium comprises first and second Ramanamplifying media cascaded to each other, and said first pump sourcecomprises first and second light sources for pumping said first andsecond Raman amplifying media, respectively.
 21. A method according toclaim 20, wherein said first Raman amplifying medium comprises apositive dispersion fiber and a negative dispersion fiber cascaded toeach other, and said second Raman amplifying medium comprises adispersion compensating fiber.
 22. A method according to claim 20,wherein said first Raman amplifying medium comprises a part or the wholeof an optical fiber transmission line.
 23. A method according to claim13, wherein said optical amplifying medium comprises a rare earth dopedfiber.
 24. A method according to claim 23, wherein said rare earth dopedfiber contains at least one rare earth element selected from Er, Nd, Tm,Pr, Yb, and Dy.
 25. A device comprising: a first optical amplifying unitcomprising a Raman amplifying medium and a first pump source for pumpingsaid Raman amplifying medium; a second optical amplifying unit opticallyconnected to a rear stage of said first optical amplifying unit, saidsecond optical amplifying unit comprising an optical amplifying mediumand a second pump source for pumping said optical amplifying medium; andmeans for detecting the spectrum of spontaneous Raman scattering lightgenerated in said Raman amplifying medium.
 26. A device according toclaim 25, wherein said spontaneous Raman scattering light is extractedfrom light to be amplified by said first optical amplifying unit on theupstream side thereof.